1. Field of Application
The following description relates generally to telecommunications systems and wireless communications systems.
2. Prior Art
A wireless network uses pilots or reference signals for channel estimation and data demodulation. For two communications devices communicating to each other, a pilot is a signal known to both devices. The transmit-device transmits the pilots along with the data to the receive-device. The receive-device uses the pilots in the received signal to estimate the channel. The data can then be demodulated with the knowledge of the estimated channel. The pilot can also be used for synchronization, timing and frequency tracking.
In LTE (long-term evolution) and LTE-advanced wireless cellular networks, which use orthogonal frequency-division multiplexing (OFDM) radio access technology (RAT), the pilots, or the reference signals as referred to in LTE, are allocated roughly uniformly over the resource elements. A resource element is one subcarrier in an OFDM symbol, and thus can be identified with the subcarrier index in frequency domain and the OFDM symbol index in time domain. FIG. 1 illustrates an example allocation of reference signals among the resource elements for a single channel.
As LTE incorporates more and more advanced features for increased data throughput, the need for more reference signals increases. For example, a 4×4 MIMO (multi-input, multi-output) will require four reference signals, and an 8×8 MIMO will require eight. With beamforming and multipoint broadcasting (or CoMP, Coordinated Multi-Point transmission, in LTE-advanced), another type of reference signal, mobile-user specific reference signal, in addition to the conventional, cell-specific reference signal, is also needed. From FIG. 1 where only one reference signal for one single channel is shown, it is apparent that as more reference signals are allocated in the reference element grid, fewer resource elements are available for useful data. It can be said that the reference signals in LTE incur the resource penalty. With the reference-signal allocation scheme in FIG. 1, it may soon reach to the point beyond which the benefits of the advanced features for increasing the data throughout no longer make up for the loss of resource elements for useful data due to the added reference signals.
Without an effective approach to providing a multitude of reference signals or pilots, the benefits of the advanced features of LTE cannot be fully realized, or may not be realized at all. Thus a strong need exists for a method, apparatus, and system that can transmit as many pilots as needed without compromising the capacity of the useful data.